Many power systems include a DC electric motor for driving a mechanical power load. For example, a mobile machine (such as a locomotive) may have a DC electric propulsion motor for driving a propulsion device (such as a wheel) to propel the mobile machine. A DC electric motor often includes a field coil (a stationary coil) and an armature (a rotating coil mounted on the rotor of the electric motor). When a DC electric motor drives a mechanical power load, if the torque produced by the electric motor becomes greater than the friction resistance torque exerted against the DC electric motor by the mechanical power load, the DC electric motor may accelerate independent of the load.
This may occur, for example, in the case of a DC electric motor driving a wheel of a locomotive if the wheel torque applied to the wheel by the electric motor becomes greater than the adhesion torque resulting from the adhesion of the wheel to the associated rail. In many circumstances, without knowing the precise value of the adhesion torque between the wheel and the rail, the operator of a locomotive may try to maximize acceleration by attempting to increase the wheel torque generated by the DC electric motor to a high percentage of the adhesion torque. In doing so, the operator may sometimes increase the wheel torque beyond the adhesion torque. Additionally, the adhesion torque between the wheel and the rail may abruptly decrease at some points on the rail for various reasons, which may also cause the wheel torque to exceed the adhesion torque.,
When the wheel torque exceeds the adhesion torque, the wheel may begin to slip on the rail, and the resulting change from a static coefficient of friction to a dynamic coefficient of friction may significantly reduce the adhesion torque, causing the wheel to accelerate. The more rapidly the wheel accelerates after losing traction, the more time and corrective action it will take to regain adhesion between the wheel and the rail.
When the wheel slips, the rate at which the DC electric motor accelerates depends on the torque generated by the DC electric motor. The more rapidly the torque generated by the motor decreases, the less rapidly the electric motor will accelerate.
The torque generated by a DC electric motor depends in part on the net voltage across the DC electric motor, which equals the difference between the external voltage supplied to the DC electric motor and the magnitude of the opposing “back EMF” generated internally by the DC electric motor. If the external voltage remains constant, increasing the back EMF decreases the net voltage across the DC electric motor, thereby decreasing the current through the armature, which decreases the torque generated by the DC electric motor. The back EMF generated by a DC electric motor equals the product of the electric current in the field coil, the speed of the DC electric motor, and a constant. The positive correlation between the speed of the DC electric motor and the back EMF creates a tendency for the back EMF to increase with increasing speed.
However, the configuration of a typical “series” DC electric motor produces an effect that partially offsets the positive correlation between the back EMF and speed. A typical series DC electric motor has its field coil and armature electrically connected in series, resulting in the field coil always carrying the same magnitude of electric current as the armature. Because of this, any increase in the back EMF would cause a decrease in the electric current in both the armature and the field coil, which would have the effect of decreasing the back EMF. With the current in the field coil forced to decrease at the same rate as the current in the armature and thereby largely offsetting the effect of the increased speed on the back EMF, the back EMF generated by a typical series DC electric motor increases somewhat gradually with increasing speed. Accordingly, when the mechanical load on a typical series DC electric motor decreases abruptly, the motor may accelerate to a very high speed relatively quickly.
U.S. Pat. No. 3,930,189 to Smith (“the '189 patent”) discloses a system for suppressing the torque produced by a series DC electric traction motor when a wheel connected to the motor slips. The system of the '189 patent includes an alternator, a plurality of current transformers having primary windings connected to the alternator, and a main power rectifier connected to the primary windings of the current transformers. The armature and field coil of the DC electric traction motor are connected between the terminals of the main power rectifier. If the wheel connected to the DC electric traction motor slips and the electric current flowing through the armature and the field coil begins to drop, the secondary windings of one of the current transformers supplies auxiliary current through the field coil separate from the armature.
Although the '189 patent discloses a system for generating greater electric current in the field coil of a series DC electric motor than in its armature, certain disadvantages persist. For example, the '189 patent does not disclose any way of generating greater electric current in the field coil than in the armature other than by supplying electricity to the field coil with an external power source.
The power system and methods of the present disclosure solve one or more of the problems set forth above.